1. Field of the Invention
The invention relates to composite structures including fibers within a matrix.
2. Description of the Prior Art
Composite structures having structural-strength fibers embedded within suitable matrices have developed extensively within the past two decades. Specific examples of such composites include structures consisting of graphite fibers within a polymeric or metal matrix. Graphite structures have been successfully used in applications requiring high modulus, high strength, flexibility, and light weight.
Recently, the rapidly developing technology of optical fibers has been combined to some degree with the earlier technology of structural strength composites. In a typical case a small number of optical fibers are introduced into existing composite structures for the purpose of affording the opportunity of examining optical signals transmitted by them, thereby to obtain data concerning the condition of the composite structure, specifically stresses and strains therein as well as breakage or deformation of the optical fibers themselves. Such composites including optical fibers are commonly known as "smart skins" and are often employed in connection with structural components of thin configuration, such as are found on the wings of aircraft.
Existing smart skin structures have, however, a number of disadvantages and drawbacks. Among these are the fact that in such existing structures a relatively limited number of optical fibers have been embedded in the matrix, with correspondingly sparse results in terms of data concerning the condition of the composite. In addition, the optical fibers may yield erroneous values for the condition of the composite since they do not bond to the matrix to the same degree or in the same manner as the strengthening fibers. Under such circumstances, the optical fibers may record only the stresses around themselves, which may be significantly different from those in the composite as a whole.
A further drawback of existing structures includes the possibility that the optical fibers may debond from the matrix thereby causing erroneous data to be produced, since debonding relieves the optical fibers of flexural strain. Also, optical time domain reflectometry yields relatively inferior resolution with respect to isolating the precise areas of high strain in a distributed sensing mode, which examines stresses at a series of positions along the fiber. Additionally, complexities in data processing result from multiplexing of output signals from a plurality of embedded optical fibers, reasonable resolution, for example, in a composite of 1 m by 10 cm 1 cm requiring approximately 1000 fibers; thus, connection of smart skins of this type to data processing systems requires extra data processing power.
Therefore, there has been a felt but unfullfilled need for a sensored composite structure including optical fibers providing the capability of monitoring strain or other parameters in the bulk composite structure while at the same time avoiding undue complexity in signal monitoring and still providing structural strength.